Color Tuning in Rhodopsins: The Origin of the Spectral Shift between the Chloride-bound and Anion-free Forms of Halorhodopsin

• Authors: Mikhail N. Ryazantsev, Emory University; Ahmet Altun, Fatih University; Keiji Morokuma, Emory University
• Journal Title: Journal of the American Chemical Society
• Volume: Volume 134, Number 12
• Publisher: American Chemical Society | 2012-03-28, Pages 5520-5523
• Type of Work: Article | Post-print: After Peer Review
• Publisher DOI: http://doi.org/10.1021/ja3009117
• Subjects: Chemistry, General

Abstract

Detailed knowledge of the molecular mechanisms that control the spectral properties in the rhodopsin protein family is important for understanding the functions of these photoreceptors and for the rational design of artificial photosensitive proteins. Here, we used a high-level ab initio QM/MM method to investigate the mechanism of spectral tuning in the chloride-bound and anion-free forms of Halorhodopsin from Natronobacterium pharaonis (phR) and the interprotein spectral shift between them. We demonstrate that the chloride ion tunes the spectral properties of phR via two distinct mechanisms: (i) the electrostatic interaction with the chromophore that results in the 95 nm difference between the absorption maxima of the two forms and (ii) the induction of the structural reorganization in the protein that changes the positions of charged and polar residues and reduces this difference to 29 nm. The present study expands our knowledge on the role of the reorganization of the internal H-bond network for the color tuning in general and provides a detailed investigation of the tuning mechanism in phR in particular.